Wireless communication relies on a wireless medium, such as a radio channel, to convey information from a first device to a second device. An example of a wireless communication system is a cellular communication system comprising cells controlled by base stations, wherein mobile stations, also known as mobiles, roam in cell coverage areas of cells. A mobile may change attachment from a first cell to a second cell as it moves around in the overall coverage area of the cellular communication system. A change in attachment may be referred to as a handover or handoff. In handover, a mobile may be handed over from a source base station to a target base station.
In some cellular communication systems, a mobile may communicate with more than one cell at a time. For example, where a mobile receives transmission from more than one cell, it may be considered to be in a soft or softer handoff situation. In some systems, a carrier aggregation may be defined comprising carriers from more than one cell to increase a throughput rate of data to a mobile.
In some cellular communication systems, base stations are configured to broadcast information to facilitate mobiles' attachment to cells controlled by the base stations. Since the information is broadcast, it may be received even by mobiles not attached to the cells. The broadcasted information may comprise, for example, cell identities, network identities and other information useful for attaching to the cells.
Some cellular communication systems comprise base stations which are connected to base station controllers. The base station controllers may be connected to a core network. Some cellular communication systems comprise base stations that are directly connected to the core network. The core network may be configured to provide network-level functionalities such as subscriber information management, access authentication, billing, switching and gateway connectivity to further networks, such as for example the internet. Base stations, and base station controllers in systems where they are present, may be collectively referred to as a radio-access network. The radio-access network may in general be configured to provide connectivity for mobiles in the coverage area of the radio-access network. The radio-access network may be configured to rely on the core network for network-level functionalities.
Some cellular communication systems comprise radio-access networks with a hierarchical structure in that the radio-access networks comprise two kinds of base stations, macro base stations and local access points. Macro base stations may perform a role similar to base stations in cellular communication systems with only one kind of base station. Local access points may be controlled by macro base stations, and may be configured to enhance access, throughput or quality in a coverage area that is geographically more limited than a cell coverage area of a cell controlled by a macro base station, for example. Radio-access networks with a hierarchical structure, or cellular communication systems comprising such radio-access networks, may be referred to as heterogeneous networks.